The science of triathlon fueling

Credit: Gray Mortimore/Allsport
During the 1995 Hawaiian Ironman, Paula Newby-Fraser skipped a few pit stops and ran out of gas on the bell lap. She sat down on Alii Drive, literally within sight of the finish line, and zoned out as Karen Smyers won the race.

During the last eight miles of the run, Newby-Fraser was focused, but was still being caught by Smyers. So she passed up a few aid stations to buy herself some precious seconds. She thought she could get to the finish on desire. She can tell you now that desire alone cant do it youve got to have gas, too.

Your body is like a jet airliner, equipped with several fuel tanks. Much of your fuel is stored as glycogen, and while most of your glycogen resides in your muscles ready for use, another quarter to a third is held in your liver.

Then there are your fat reserves, and these are called upon during long-distance racing. Youll always burn a certain amount of fat during exercise, but the rate doesnt rise as the intensity rises.

It also, by the way, doesnt go down. There is a misconception that high-intensity effort isnt fat-burning, when in reality it is. What happens is that your rate of glucose consumption increases as your intensity increases, which means that fat, as an overall percentage of fuel used, goes down.

Fat burns in a carbohydrate fire, it is often said, and sugars are a necessary component of fat metabolism. When you bonk, you are, of course, out of the sort of fuel that is most efficiently metabolized meaning sugars. Whats more, in the absence of sugars your body will not be able to metabolize fat and instead will seek out other sources of fuel.

The bodys cortisol levels increase during exercise and cortisol is catabolic, meaning it breaks down tissue. One of the bodys uses for cortisol is to catabolize lean body mass (i.e., muscles) for use in a process called gluconeogenesis, in which the body converts protein to a useable source of fuel.

The message is well-presented in Powerbar commercials: Dont bonk.

There is another reservoir of fuel you carry around with you. Its called "whatever is sitting in your stomach." This is important, and personally, Id recommend eating as hearty a breakfast as you can tolerate on the morning of a long triathlon. You shouldnt worry about any negative digestive effects from a nice-sized breakfast because youre probably not going to start the race sooner than two hours after finishing your meal, and youre not going to be going that hard during such a long race anyway.

All of these fuel reservoirs will take you through three hours' worth of Ironman-effort-level work, or perhaps four in a best-case scenario. That said, you dont want to go through your glycogen stores early in the race, as youll need them later. This is because the critical factor is not how much you ingest during the race but what you ingest. And even more important is your burn rate and your absorption rate. Your burn rate is going to be higher if you go harder. And remember, youre burning about the same amount of fat riding at a slower pace as you are riding at a faster one, and all the additional fuel required to go faster is much-needed carbohydrate.

This is important for two reasons. First, when you run out of carbohydrate, as we discussed, you really run out of two fuel sources: carbohydrate and fat. And when youre riding harder youre going through carbohydrates very quickly.

Second, you can never re-absorb as much as you burn. Youll burn 800 to 1,000 calories an hour during an Ironman race. You can absorb only about 275 calories an hour, or perhaps more if youve got a particularly efficient system for gastrointestinal uptake. (Mark Allen believes, it is reported, that he can uptake more than 400 kilocalories an hour and as much as 500 kilocalories an hour.)

Either way, you dont want to wait until youre out of glycogen before you replenish. Considering this, you can see why people literally run out of fuel on the course.

Good fueling doesnt start when you get on your bike, or even at breakfast before the race. It begins days before the event. Much has been written about carbohydrate loading, and Ill defer to others for recipes and protocols for this. Proper pre-race hydration is a bigger concern. You need to enter a long race fully hydrated. That means drinking a lot of water in the two or three days prior to the race and refraining from peeing it right back out because youre taking in diuretics at the same time. So be careful of drinking too much coffee or tea, not to mention alcohol.

My wife, JulieAnne whos been second place among women in the Hawaiian Ironman has an extremely low salt intake in her regular diet. Normally, of course, one would not want to take in too much sodium. But her intake is so low that she is sometimes in danger of becoming dehydrated early in a race. Her exceedingly low sodium intake makes it difficult to keep cellular-level water at a decent pre-race level.

So it might not be a bad idea for those who also have a very low salt intake to drink fluid replacement instead of clear water in the days prior to the race in order to increase their sodium intake and pack their cells full of water and sodium.

While were on the subject of sodium: Your body regulates its sodium based on how youve conditioned it. When you see a person cross the finish line of a race with those white salt deposits all over their clothing, its a fair bet they have a pretty high amount of daily sodium intake. This is especially true in America. While Europeans average about 6 grams of sodium a day, Americans take in about 10 grams a day.

We have a gaggle of dogs in our household, and one of them loves the taste of salt. When my wife and I come in after a long run or ride I have to beat this dog off with a stick or subject myself to 20 minutes of licking and since I dont beat the dogs, I have to sit for the latter. It isnt because the dog likes me more than Julie, but because my normal salt intake is much greater than that of my wife, which means I excrete a lot more salt in my sweat. Therefore, in a long race my need for salt is going to be much greater.

Somebody who processes salt this quickly can go through as much as 5 grams of salt an hour in a long, hot race. That means two things. Id be a better long-distance athlete if I didnt metabolize so much salt which means Id be well-advised to cut down my regular salt intake, and perhaps some months later I will have trained my body not to sweat out so much salt. And second, there is no possible way my body will take in as much salt as Ill need with any fluid replacement drink. The average gel only has 25 to 50 milligrams of sodium, and even a salt tablet will only have 250 milligrams.

Some pro athletes have ingested a couple dozen salt tablets during a long, hot race like Hawaii.

If you cramp late in long rides or if you have excessively salty sweat, you might consider supplementing with salt tablets during your race. But like everything else written here, this isnt something you can simply try the day of the race. You should practice your fueling plan during training, and if you think youll need extra salt during the race, you should practice this as well.

All this is well and good, you might say, but its pretty theoretical. If it is this easy, why do I see athletes who do this for a living bent in half by the side of the road instead of clipping off six-minute miles?

Good point. Sometimes things dont go as planned. Those pros are just as interested as you in avoiding such disagreeable outcomes to their races. In part two of this series Ill go through the race step by step, sharing some of the tricks top athletes use to reach the finish line whole and happy.

Dan Empfield is founder and former owner of Quintana Roo, Inc. He now publishes two popular online portals for triathletes, found at and He can be reached at

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